An Advent calendar is a symbol of the holy season of Advent, celebrated in December near Christmas, another holiday season.

The calendar consists of 24 doors or pockets, with door one being opened on December 1 and door twenty-four being opened on Christmas Eve. Each compartment can either show a part of the Nativity story and the birth of Jesus, or can simply display a piece of paraphernalia to do with Christmas (e.g. bells, holly). Such is the closeness of the two holidays that Christmas is now integrated into many Advent calendars.

Many calendars have been adapted by merchandisers and manufacturers to include a piece of chocolate or a sweet behind each compartment, aimed at children. These have often been criticised for not relating to the Nativity and simply cashing in on Christmas sales.

The number of doors can also increase to 25 or 26 to cover Christmas Day and Boxing Day, and further to 31 or 32 to include New Year's Eve and New Year's Day. This latter act was particularly evident over December 1999, counting down to January 1, 2000 and the start of the third millennium.

The Advent calendar is normally of standard dimensions, but can be found in other shapes, such as a model of a house. There are alternative forms of Advent calendar, such as those made from felt or other material, or a chain of candles that can be lit day by day. The German city of Dresden has a giant calendar built into a fairytale castle on its Christmas market, the Striezelmarkt.

Julkalender, a Swedish variant, is based on a TV- or radio-show series that runs for 24 days for children. It generates great attention in the media. Typically, in connection with the show, the calendar door is opened.

The joulukalenteri, a Finnish variant, is based on a TV series that runs for 24 days for children but it's also suitable for older audience because of its great humor and ideas.